Tankless the only option? - MultiHead Shower

Hi,
I'm about to install 2 Rinnai R94LSi tankless water heaters (with a re-circulating system) to handle a new bathroom remodel I'm working on.
The new shower will have 4 Kohler water tiles (2.5GPM), a hand shower (2.5GPM) and an overhead shower (4GPM). The heaters will be used for the rest of house as well (1.5 other bathrooms, std stuff, 1 sink in kitchen, 1 washer, 1 dishwasher). The new shower will use the Kohler 3/4" Thermastatic valves (though considering the DTV system as well).

This forum has me paranoid that I"m making a huge mistake, though I don't know of any tank option to get the water flow to support such a shower. My current water heater (10yrs old) supplies enough water for my wife and I with our pre-remodel shower. It takes 2min for water to get from turn on in our kitchen currently, hence the want for the recirc system. We're having the units installed by a Rinnai authorized installer to prevent installation hassles.
I should note that we're in Texas. So things don't get overly cold.

Are there tank systems that support such a setup?
Spending all this money and ending up with a cold or fluctuating shower temp would drive me nuts.

One of these units (GU32) can do 6GPM at 75F. Measuring my water this morning, it was around 60F. To get to 120F, (60F rise), it could 7GPM. So I'm still likley going to have to look at 2 units.

Has anyone here had experience or problems with a recirculating system on a tankless?

The eternal site says they don't have issues with reciruculating systems.

I'm considering a DTV setup or a Kohler Thermostatic valve. A lot of the tankless problems seem to be centered around thermostatic valve issues. I'm struggling to understand how the hot water from a tankless is any different from the hot water form tank system when it gets mixed...

Last continued education class I was in I heard the instructor talking about a new standard in pressure and temperature balancing valves. It uses a wax that reacts to temperature and pressure fluctuations and it reacts instantly. Unlike the old bi-metal temperature balancing, and the old slug type pressure balancing valves. I was wondering if these new valves are the ones they are having issues with and if anyone here has come across this new style valve.

With a tankless system, to ensure they get a temperature rise, they use a restrictor. This may or may not slow your flow until you reach the limit. The better ones also modulate the burner to accomodate both volume and temperature variations. If you exceed the available heat requirements, the output will both hit a wall with the pressure and the temperature will drop. Many valves have some troubles with both of those asymettrically changing.

Each brand has different requirements for what is necessary (or even recommended) for recirculation with a tankless system. If you don't go with a demand system that you turn on only before you want hot water, and need it instantly all the time, then you need something to buffer some hot water so the tankless systems aren't running constantly, so that adds to the cost with a tank and the extra plumbing.

Not sure what I'd do (other than I probably wouldn't have one in the first place!).

I may be looking at this wrong, but if I understand your criteria correctly, you may want to consider keeping the hot water system to the master shower independent from the rest of the house. The rest of the house sounds pretty straight forward and you could benefit from a simple, inexpensive, easy to maintain and service, straight forward solution to that part of the house.

That would free you and your plumbing contractor up to focus on how to provide a steady supply of hot water at a constant temperature to the equivalent of 4 simultaneous showers for the anticipated time of use.

I'm struggling to understand how the hot water from a tankless is any different from the hot water form tank system when it gets mixed...

Thanks for the help.

Click to expand...

The difference is the water pressure of the hot and cold side. In a tank system, the hot and cold have the same pressure, since a tank causes very little resistance flow. In a tankless system, the heater must maximize surface contact area for maximium heat exchange, which cause alot of friction and loss of dynamic pressure. This has the hot water at the shower having less dynamic pressure than the cold side. If you understand how a "pressure-balanced" faucet works, you'll understand the problem.

Buried back on page 38 of the RS94 manual you'll see the gpm vs pressure loss chart. You'll see once you get past the low flow rates (single shower), pressure loss becomes a real issue.

I'm not sure what Rinnai recommends here. If they are plumbed in series the problem would be twice as bad. If the are plumbed in parallel the pressure problem would be half, but your minimum flowrate would be double. The other option would be to raise the temperature so you would use less gpm, but that would create a scald hazard.

I'm sure the manufacturer must have considered this situation, but balancing multiple units in parallel would be difficult.

My guess is that they run them in series and use a restrictor sized for the combined output capability...using the second unit (closest to the destination) with a flow sensor, and only triggering the first one when the second one's output cannot keep up with the demand.

I should have been more explicit on at least two counts. Two R94LSi's plumbed in series will not work properly; they should be plumbed in parallel and linked using the "EZ Connect" feature. At least that's what Rinnai will tell you.

Tankless is not the only option in a heavy residential installation. I would think a commercial tank model or two could also meet his demands. Perhaps the OP could pose this question to over in the tank forum.

I'm having a plumber come in to give a quote tomorrow. Unfortunately, it still looks like I might have to have 2 systems to handle a 60F rise for 10-12GPM.
From what I can tell, it solves the solves the cold sandwhich and startup temp problems and prioritizes flow over temp, which I can live with more than having my thermostatic valve freeze me out.

Unfortunately, there aren't many reviews out there as far as I can tell. That means I'm eating the manufacture's cake and letting him tell me how good it is.

Anyone have experience with this kind of Hybrid?
Is Eternal the only hybrid like this? I saw a blurb about an AO Smith hybrid, but it looks completely different.

Can you tell the forum more about your balancing criteria for system selection? Are tank type systems out of the question, for whatever reason? Your hot water demand can be met by any number of means; it would help to know a little bit more what's important to you.

Only one of the two hand shower or shower head needs to be on at any given time. So a total of 12.5GPM.
For this experience, I do not want to worry about not having enough water pressure or having cold bursts of water intermingled with my shower, especially having a cold burst because someone else in the house does something as simple as turning on a faucet.

I would prefer to be able to take a standard shower (non-body spray) and do laundry, run a dishwasher, or have another person taking a shower elsewhere and experience no ill effects.

Tanks are not out of the question, though by my math a 90 Gallon tank at 75% mix will only get a single 12min shower or so.

Our current water heater is in our attic (Texas style). My wife is not a big fan of having mega gallons of water in our attic due to paranoia over a leak or spill (bad experience).

My water temp this time of year is 60-65F. My water pressure at the faucet outside is 55-65PSI. This leads me to the 60F rise (60->120) that I use to look at all of the GPM data on the tankless units.

I keep having to turn my computer over to the IT guys so pardon the brevity of my posts. A single 100 gallon Bradford White EF100t (commercial) has a first hour delivery capacity of 569 gallons at 60F rise, a recovery rate of 499 GPH, and a 3 hour average of 523 GPH and is supplied by a 3/4 inch gas line and requires a 3 or 4-inch vent. That's overkill, of course, but the point is there's more than one way to skin a cat if all your looking for is back to back over the top showers. Also consider that the use of tempering valve will extend the first hour delivery capacity of a more modest water heater.

The tankless units I like that are similar to the hybrid heaters is the Navien Tankless "A" Models they have a built in recirculation pump and a mini buffer tank. Duel stainless heat exchangers, PVC venting 98% efficiency.

These units really impressed me when I seen them installed and at the trade shows. The built in recirculation pump has the ability to recirculate the water with in itself, to help eliminate the "cold-water sandwich" if you do not have the piping available for external recirculation.http://america.navien.com/KD_eng/internal.html

The cost I have for 2 x R94Lsi Rinnai's + Recirc system (includes 12 Gallon electric tank) 30% cheaper than a Single GU32 system with recirc installed.

Given the cost, I think I'm going to end up doing the Rinnai's. I'll figure I'll have to spend maintenance for flushing once a year, replacement of recirc tank every 4 years or so. Even with that, the Rennai is going to be cheaper.

With those facts and choices, it seems like a no brainer, provided you have the space. Consider:
1. You would need to maintain the GU32 as well.
2. Odds are against both Rinnai's failing simultaneously. So, if one goes bad, you still have one to limp along on until you can repair/replace the broken unit.
3. If your recirc tank goes bad, it's easy and cheap to replace. Should be DIY simple. What brand and model recirc tank are you planning to use?

I'm gravitating toward a similar design, but probably involving 3 tankless units in parallel instead of two, provided the minimum flow rate does not increase by 3x. I would guess the manifold controller lights up one tankless unit until the flow rate is 2x or 3x the minimum, at which point it would light up the second and third unit respectively and then load balance thereafter. Not sure if I'll use Rinnai or a tankless (still to be identified) whose heat exchanger enables the pottable water to be physically isolated from the hot water being recirculated through my hydronic heating system.

As an aside, the tax rebates on solar are looking pretty good right now (even with installation, I'm told it's almost zero net cost), so I might add that into the mix. Since you're in Texas, you might want to consider it as an add-on also. It was actually the Rinnai dealer who was trying to sell me on solar.